Continued demand for higher wireless data rates drives advances in multiple aspects of wireless communications systems and methods. Relevant to this invention is beamforming at an array of antenna elements. In such an array, individual antenna elements are used to beamform signals to and from the transceivers connected to those antenna elements so as to add antenna diversity to the wireless signals. Antenna diversity enables the receiver to capture, and the transmitter to emphasize, different wireless pathways that a signal follows between sender and recipient. By resolving these multi-paths and adding to them with MIMO techniques, a fading signal can be more reliably decoded so that less bandwidth is required for re-transmissions and error correction/control. Different active sets of antenna elements in the array may be used at different times and for different signals, so in an ideal case the choice of the active antenna element set is dynamic. Currently, arrayed antenna systems are typically disposed at fixed terrestrial locations such as wireless base stations of a cellular/PCS network, land-based military sensing stations, and in orbiting satellites.
An important consideration in arrayed antenna elements is calibration, specifically phase and power. For a spread spectrum signal, the phase of a signal received at different antennas may vary by the time it reaches the receiver for despreading and decoding, due to different electrical path lengths from antenna element to receiver. These phase errors need to be corrected for proper despreading in a correlator. Further, the signal power must also be closely matched at the receiver while the signal is still spread so that both versions can be readily recovered. Similar considerations apply to beamformed transmissions. Because there are multiple antenna elements and the active set of antenna elements changes for different signals and conditions, the problem of calibration is highly complex. The state of the art has evolved several ways to deal with this calibration problem.
Some prior art approaches have favored the use of directional couplers to find the relative phase and amplitude differences for signals at different antenna elements or active sets of elements (e.g., a sub-array), as detailed in the background section of the incorporated reference. Such phase-accurate RF coupling and connection networks impose a constraint in manufacturing of arrayed antennas because the prior art tends to rely on close tolerances for the physical length (of coaxial cable, microstrip lines, etc.) between the antenna port and the calibration port. A costly measurement system during manufacture is also necessary to account for the true propagation speed of the conductive media between those ports, which typically varies over a fairly broad range for any arbitrary manufacturing lot, so accuracy of the phase electrical length cannot rely on physical length of the conduit alone. In PCB materials used in the antenna elements, the relative dielectric constant ∈r also typically varies between the x and y directions, so that the signal propagation speeds and hence the electrical lengths vary as a function of direction. However, phase accuracy is a key parameter in effectively using an antenna array system.
In general, it is mandatory for adaptive antennas and beneficial for MIMO antennas that phase and power distribution over the entire antenna aperture be known and controlled, otherwise the desired radiation pattern will not be formed. This is also a useful feature for more simple antenna structures if there is a need to tailor the radiation pattern during or after installation. Such would enable integrating the radios/transceivers and the antenna(s) to a common unit and remove the need of having several different antenna versions, which would simplify manufacturer's logistics. Two antenna terms are distinguished: adaptive antennas track mobile user equipment and steer power toward the specific mobile users, and active antennas are antenna radiators that have the radio or RF functionality built in. These teachings are advantageous for both types, which may be also combined into an adaptive active antenna.